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Duke Elder - Anatomy 2: Structure of the eye > Sclera > Flashcards

Sclera Flashcards

1
Q

What are the 2 main functions of the sclera?

A
  1. protects intraocular contents
  2. maintains shape of globe when distended by intrinsic intraocular pressure
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2
Q

What happens to the globe shape during contractino of the extraocular muscles and why is this remarkable?

A

it maintains its shape; the EOM tendons insert on the scleral surface

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3
Q

What is different about the corneoscleral envelope (fibrous coat) in infants?

A

it is not mature and can increase in size (buphthalmos in congenital glaucoma)

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4
Q

What is the vascularisation like of the sclera?

A

relatively avascular

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5
Q

What does the viscoeleastic nature of the sclera (great tensile strength, extensibility and flexibility) permit in terms of changes in intraocular pressure?

A

it allos only limited distension and contraction to accommodate minor variations in IOP

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6
Q

At what point is the sclera thickest and what is its thickness here?

A

posteriorly - 1mm thick

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7
Q

At what point is the sclera thinnest and what is its thickness here?

A

behind the insertions of the aponeurotic tendons of the extraocular muscles: 0.3-0.3mm

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8
Q

What covers the sclera a) posteriorly and b) anteriorly?

A
  • a) fascia bulbi
  • b) conjunctiva
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9
Q

What is the struture of the sclera?

A

consists of dense, irregular connective tissue comprising extracellular matrix and marix-secreting fibroblasts

matrix contains collagen (type I mainly, also III, IV, V, VI, VIII, XII, XIII)

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10
Q

How does the structure of the cornea differ significantly from the sclera? 2 key ways

A
  1. the scleral collagenous lamellae are irregularly arranged
  2. scleral lamellae are interspersed with elastic fibres
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11
Q

What is the principal type of collagen present in the matrix of the sclera?

A

collagen type I

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12
Q

What is the structure of the elastic fibres present interspersed in the scleral collagenous lamellae?

A

they consist of an elastin core surrounded by longitudinally arranged microfibrils composed of a number of glycoproteins including fibrillin

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13
Q

What are the microfibrils around the elastic core that form elastic fibres in the sclera formed of?

A

glycoproteins including fibrillin

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14
Q

What are 5 factors that contribute to the opacity of the sclera (compared with the transparent cornea)?

A
  1. irregular arrangement of the collagen fibres
  2. variable fibre diameter (25 -250 nm)
  3. variable and irregualar fibrillar spacing
  4. higher water content
  5. reduced coating of GAGs on collagen fibres
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15
Q

What proportion of proteroglycans and glycosaminoglycans (GAGs) are there in the sclera compared with the cornea that contribute to its opacity?

A

sclera contains 1/4 of the content of the cornea

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16
Q

What are the 2 most abundant proteoglycans in the sclera?

A
  1. dermatan sulphate
  2. chondroitin sulphate
17
Q

What is the cause of buphthalmos?

A

the corneoscleral envelope of children with congenital glaucoma responds to raised IOP by irreversibly stretching, owing to the immaturity of the collagen fibres –> produced characteistically enlarged buphthalmos (ox-eye) of this condition

18
Q

What is shown in the image?

A

collagen bundles in the sclera ( x 7000)

19
Q

What is the cause of yellowing of the eyeballs?

A

bilirubin deposition in the conjuctiva nad not the sclera

20
Q

What can lead to a blue tinge of the sclera?

A

abnormal thinning of the sclera, such as occurs in some connective tissue disorders e.g. Ehlers Danlos syndrome

21
Q

What is staphyloma?

A

bulging - can be caused by localised thinning of the stromal collagenous layers

22
Q

Why do infants have blue sclera?

A

underlying choroidal pigment showing through the thin collagenous stroma

23
Q

Why do elderly individuals sometimes have a yellowish hue to the sclera?

A

fat deposition in sclera

24
Q

What is the function of the collagen fibrils in the sclera?

A

take up tensile force and are aligned with the direction of greatest tensile strength

25
Q

How can the arrangement of scleral collagen be studied?

A

using the ‘split line’ technique

26
Q

What formation are the collagen bundles in the outer sclera arranged in?

A

whorls, loops and arches, particularly around the muscle insertions and optic nerve

27
Q

What formation are the collagen bundles in the internal aspect of the sclera arranged in?

A

rhombic pattern

28
Q

Where does the sclera extend between?

A

from the limbus anteriorly to the lamina cribrosa posteriorly

29
Q

How are the collagen fibrils in the sclera arranged at the lamina cribrosa?

A

in circles or figure-of-eight patterns

30
Q

What structures traverse the sclera? 3 things

A
  1. anterior ciliary arteries
  2. vortex veins
  3. long and short posterior ciliary arteries
31
Q

What are the 3 histological layers of the sclera?

A
  1. lamina fusca
  2. stroma
  3. episclera

Duke Elder - Anatomy 2: Structure of the eye (14 decks)

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